More stable JSON: Sort crossRef array #243

There is a use case that downstream users of license list data need to periodically update license JSON files (see #146).

Currently, the key order in current JSON is fairly stable, by order of creation.
The issue is the order of crossRef array members (see https://github.com/spdx/Spdx-Java-Library/pull/429/changes)

We may like to sort the crossRef array by the order key (integer) to make the JSON output more stable.